The invention relates to underground coal gasification and, more particularly, to a controlled retracting injection point system for in situ coal gasification.
Coal gasification methods have been used to produce combustible fuel from coal. When coal is heated in the presence of oxygen and steam it gives off a mixture of combustible gases which can be refined and purified and used as fuel. These product gases are environmentally more advantageous than coal since they burn cleaner, producing less air pollution, and are easier to transport. Above ground gasification processes are available but require expensive and difficult mining and transportation before surface processing.
It is more advantageous to gasify coal in situ by chemically reacting the coal underground to produce combustible product gases. Boreholes are drilled into the seam, and the coal is ignited, and oxidizing gas and steam are pumped down through an injecting well to support combustion in the underground reaction zone. The coal is partially oxidized producing gas of low or medium heating value. The hot product gases flow through a channel in the coal seam and are removed through a production or recovery well. For the process to occur a permeable path through the coal must be provided between the injection and production well to permit the high volume gas flow that is required.
Linking methods include countercurrent or reverse combustion, directional drilling and electrolinking. Countercurrent or reverse combustion linking is the most commonly used technique for enhancing the permeability of the coal bed. Air is forced into the injection well and flows to the production well through natural fissures in the coal bed. The coal at the bottom of the production well is ignited and a burn front is drawn by conduction to the source of oxygen, charring a narrow channel countercurrent to the flow of air. The directional drilling method produces a gasification channel through the coal by drilling along a coal seam at varying angles and intersecting the production and injection wells. The link is established at the bottom of the coal seam so that as the gasification process progresses coal falls into the void producing coal rubble with a large surface area for coal-gas reactions.
In practice, the underground coal gasification process is subject to various problems that may make it difficult to maintain and control an efficient long-term operation. A major problem is the need to move the injection point, where the combustion supporting air or oxygen from the surface is fed into a coal seam, to new areas of unburned coal as the burn progresses. With the standard arrangement of injection well, linking channel, and production well, the burn zone geometry is constantly changing as the cavity grows around the injection well toward the production well and up to the roof rock. Roof collapse fills the cavity with inert material, providing an opportunity for oxygen to bypass the reaction zone and oxidize the product gas lowering its quality. Heat loss increases as more and more roof material is exposed with consequent lowering of the heating value of the product gas. Vertical injection wells are subject to an extremely harsh environment of high temperatures, corrosive gases and massive mechanical forces from rock motion.
Experiments conducted at Hoe Creek indicate that maintaining the injection point at a low position in the coal seam is essential for obtaining good gas quality and high resource recovery. However, maintaining a low injection point is very difficult with a vertical injection well. Accordingly, a system is desired which provides a more constant burn geometry and a seam bottom injection point.
U.S. Pat. No. 4,334,579 to Gregg issued June 15, 1982 describes methods for coal gasification, including several embodiments implemented from the coal face utilizing a withdrawable or degradable injection pipe, in which the injection point is moved sequentially around the perimeter of a coal removal area to sweep out the area and maintain gas-coal contact.
U.S. Pat. No. 3,563,606 to Sears issued Feb. 16, 1971 discloses a method of coal gasification utilizing a withdrawable inlet pipe.
It is an object of the invention to provide method and apparatus for the controlled retraction of the injection point in an underground coal gasification process.
It is another object of the invention to provide method and apparatus for the controlled destruction of the injection pipe in an underground gasification process to controllably move the injection point.
It is another object of the invention to provide a method and apparatus for igniting coal in an underground coal gasification process.